Among various thermal transfer recording systems is a known sublimation transfer recording system which uses a sublimable dye as a coloring material and employs a thermal head heating in response to a record signal to transfer it to an image-receiving sheet, thereby obtaining an image. With this recording system, the coloring material is a sublimable dye capable of density gradation. Thus, the resulting image is very sharp, excellent in halftone color reproduction, and comparable in image quality to silver salt photography.
Thus, the sublimation transfer recording system is utilized widely. Its uses, for instance, cover proof sheets, digital image outputs such as CAD/CAM and CG, outputs from various medical analyzing or measuring instruments, such as CT scans or endoscopic pictures, substitutes for instant photographs, outputs such as face photos on cards, including ID cards, photo-montages in amusement facilities such as recreation parks, and photographic records.
The thermal transfer image-receiving sheet for sublimation transfer (hereinafter referred to as the image-receiving sheet) used in a variety of fields generally comprises a coloring material receptive layer formed on a substrate.
Needless to say, major requirements for the image-receiving sheet are high print sensitivity and stability to curl before and after printing. Thus, a substrate used for the sheet is paper, or a laminate having a resin layer containing fine voids inside, such as a plastic film or synthetic paper, laminated on one or both surfaces of paper.
Recently, as the above-described uses diversify and broaden, the market's demand for the appearance of the image-receiving sheet, such as smoothness, has become intense. Concretely, smoothness enough to be free from visually noticed undulations or irregularities has been demanded.
When the substrate of the aforementioned structure is used, however, irregularities or undulations probably associated with paper contained in the substrate appear on the surface. Thus, the desired smoothness has not been achieved easily, and the quality appearance of the image-receiving sheet has been impaired.
To solve these problems, extensive studies have been conducted. Japanese Laid-Open Patent Publication No. 227172/94, for example, describes paper with a smoothness of 20 to 120 seconds (Beck tester). Japanese Laid-Open Patent Publication No. 227173/94 describes defining the thickness nonuniformity index of paper in the direction of paper making.
These parameters are suitable for expressing tiny irregularities measuring about several tens of micrometers to 1 mm which are concerned with a lack of image or print density nonuniformity during printing. This is because the resolution of the thermal head, the device of the sublimation transfer recording system, is 200 to 300 dots per inch and conforms to the above range.
However, those parameters are not necessarily suitable for detecting or expressing the undulations or irregularities extending over several millimeters or more that affect the appearance and quality. Even paper which fulfills the required smoothness or thickness nonuniformity index has still been unsatisfactory in visual undulations or irregularities.
The present invention has been accomplished under these circumstances. It aims to solves the problem of tiny irregularities on the paper surface and the problem of undulations or irregularities as large as several millimeters that are associated with appearance and quality. By this solution, the invention intends to provide a thermal transfer image-receiving sheet excellent in print sensitivity and stability to curl, free from visually noticed undulations or irregularities on the surface, and having high smoothness and a quality appearance.